J. Phys. Chem. A 2006, 110, 10309-10315
10309
Quantum Chemical Characterization of the Vertical Electron Affinities of Didehydroquinolinium and Didehydroisoquinolinium Cations John J. Nash*,† and Hilkka I. Kentta1 maa Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907
Christopher J. Cramer*,‡ Department of Chemistry and Supercomputing Institute, UniVersity of Minnesota, Minneapolis, Minnesota 55455 ReceiVed: May 10, 2006; In Final Form: June 26, 2006
Vertical electron affinities (EA) are predicted for the lowest energy singlet states of the 21 didehydroquinolinium cation isomers and the 21 didehydroisoquinolinium cation isomers, as well as the doublet states of the seven dehydroquinolinium cation isomers, the seven dehydroisoquinolinium cation isomers, the seven Nmethyldehydroquinolinium cations, and the seven N-methyldehydroisoquinolinium cations, by using density functional theory. For the monoradicals, the calculated EA of the radical site depends only on the distance from the (formally charged) nitrogen atom, and is reduced by 0.14-0.24 eV when the NH+ group is replaced with an NCH3+ group. Nearly all of the calculated EAs for the ortho biradicals are lower (by 0.04-0.72 eV) than those for either of the corresponding monoradicals. For the meta biradicals, the calculated EAs lie either between the EAs of the corresponding monoradicals or higher (by 0.07-0.58 eV), and they are extremely sensitive to the separation (distance) between the two dehydrocarbon atoms. For the biradicals that do not have either an ortho or meta relationship the calculated EAs are all higher (by 0.02-1.93 eV) than those for either of the corresponding monoradicals. The EAs are examined to gain insight into the nature of inductive/ field and resonance effects that influence the electrophilicity of the radical site(s), which is a major factor controlling the reactivity of these types of (bi)radicals.
Introduction Organic molecules having one (monoradicals), two (biradicals), or more (polyradicals) unpaired spins are thought to play an important role in a variety of fields, including organic synthesis, development of new organic materials, and the biological activity of organic compounds.1,2 Therefore, these species have attracted significant interest over the years. Aromatic carbon-centered σ,σ-biradicals (didehydroarenes) have received especially intense attention since the discovery that 1,4-didehydroarenes are the biologically active intermediates of the enediyne class of antitumor antibiotics.3 This type of biradical can cleave double-stranded DNA via abstraction of hydrogen atoms from each DNA strand. Reactivity studies on these biradicals are hindered by their high reactivities and short lifetimes in solution.1,4 However, information on the factors that might control their reactivity is highly desirable for the development of better DNA-cleaving drugs. To improve the understanding of the properties of didehydroarenes, computations have been employed to explore various factors that might control their reactivity (e.g., singlet-triplet (S-T) gap, radical site separation, substituents, heteroatoms, charge).4,5 The reactivity-controlling role of the S-T gap for biradicals that have a singlet ground state (which is the case for most didehydroarenes) appears to be generally accepted by the scientific community.4,5a,6 However, our preliminary experimental studies on didehydro(iso)quinolinium cations have † ‡
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led to the surprising conclusion that electronic effects (due to the S-T gap) can be completely offset by polar effects in radical reactions of didehydroarenes.7 In fact, due to greater polarity, some singlet biradicals have been shown to be more reactive than related monoradicals.7 Prior to this research, nothing was known about polar effects on the reactions of organic biradicals, although these effects have been known8 for a long time to be very important in controlling the reactivity of (polar) monoradicals. While Donahue and Anderson’s ionic avoided curve crossing model9 provides a simple way to rationalize the influence of the polarity of a radical on its reactivity (expressed as the electron affinity, EA; the energy released when an electron is added to the radical), further studies are still needed to understand the influence of polarity and other factors on the chemical properties of didehydroarenes. As a first step toward a thorough investigation of the properties of didehydro(iso)quinolinium cations, we recently carried out a computational study on the S-T gaps of all 42 isomers of these molecules.10 We report here the results obtained in the second step of this series of investigationssa computational examination of the vertical EAs for the 42 isomers of the didehydro(iso)quinolinium cations and related monoradicals. Computational Methods Molecular geometries for the N-methylquinolinium cation, the N-methylisoquinolinium cation, and the 14 isomeric Nmethyldehydro(iso)quinolinium cations (MeD(I)Qs) were optimized at the multiconfigurational self-consistent field (MCSCF) and density functional (DFT) levels of theory using the
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10310 J. Phys. Chem. A, Vol. 110, No. 34, 2006 correlation-consistent polarized valence-double-ζ (cc-pVDZ11) basis set.12 For all molecules, calculations were carried out using Cs point group symmetry. The MCSCF calculations were of the complete active space (CASSCF) variety13 and included (in the active space) the full π-space for each molecule and, for each of the monoradicals, the nonbonding σ orbital. The DFT calculations were of two types. Both used the gradient-corrected exchange functional of Becke,14 which was combined either with the gradient-corrected correlation functional of Lee, Yang, and Parr15 (BLYP) or that of Perdew et al.16 (BPW91). All DFT geometries were verified to be local minima by computation of analytic vibrational frequencies, and these (unscaled) frequencies were used to compute zero-point vibrational energies (ZPVE) and 298 K thermal contributions (H298 - E0) for all species. DFT calculations for doublet states of monoradicals employed an unrestricted formalism; total spin expectation values for Slater determinants formed from the optimized Kohn-Sham orbitals did not exceed 0.76. Single-point calculations were also carried out for the ground states of the 14 isomeric dehydro(iso)quinolinium cations (D(I)Qs), the 14 isomeric N-methyldehydro(iso)quinolinium cations (MeD(I)Qs), and the singlet states of the 42 isomeric didehydro(iso)quinolinium cations (DD(I)Qs),17 using the augmented, correlation-consistent polarized valence-double-ζ (aug-ccpVDZ18) basis set. In almost all cases, these calculations were carried out for the UBLYP/cc-pVDZ optimized geometries. However, for a few of the meta biradicals,19 it was necessary to use the MCSCF/cc-pVDZ geometries because the UBLYP/ cc-pVDZ structures were bicyclic. In general, then, these electronic energies are of either the UBLYP/aug-cc-pVDZ//UBLYP/ cc-pVDZ or UBLYP/aug-cc-pVDZ//MCSCF/cc-pVDZ variety. To compute vertical EAs for the monoradicals and biradicals, single-point calculations ((U)BLYP/aug-cc-pVDZ), using the optimized geometry for each monoradical or biradical, were also carried out for the states that are produced when a single electron is added to the nonbonding σ orbital (or one of the two such orbitals) of each molecule.20 Thus, for the monoradicals (doublet ground states) these calculations were carried out for (zwitterionic) singlet states, whereas (zwitterionic) doublet states were computed for each of the biradicals (singlet initial states).21 Finally, RBLYP/aug-cc-pVDZ single-point calculations were also carried out for the singlet states of the D(I)Qs (only) using the optimized (RBLYP/cc-pVDZ) geometries in order to compute adiabatic EAs for these molecules. All MCSCF and DFT calculations were carried out with the MOLCAS22 and Gaussian 9823 electronic structure program suites, respectively. Results Geometries. Geometric information for the N-methyl(iso)quinolinium cations, the ground states of the N-methyldehydro(iso)quinolinium cations, and the (zwitterionic) singlet states of the dehydro(iso)quinolinium cations, obtained using the (U)BPW91, (U)BLYP, and MCSCF methods, is provided in the Supporting Information. For all quinolinium and isoquinolinium cations, the atom numbering scheme is indicated as follows:
In general, the (U)BLYP geometries for N-methylquinolinium cation, N-methylisoquinolinium cation, and the doublet states of the MeDQs and MeDIQs, are characterized by slightly longer
Nash et al. TABLE 1: Adiabatic and Vertical Electron Affinities (eV) for m-Dehydroquinolinium Cations and N-Methyl-m-dehydroquinolinium Cationsa 2 adiabatic vertical
vertical
3
4
5
6
m-Dehydroquinolinium Cations 6.78 6.22b 6.05 5.63b 5.31 6.31 5.77 5.59 5.19 4.90 (6.35) (5.79) (5.63) (5.19) (4.91)
7
8
5.44 5.02 (5.03)
6.04 5.60 (5.63)
N-Methyl-m-dehydroquinolinium Cations 6.07 5.60 5.45 5.03 4.76 4.87
5.37
a
Calculated at the (U)BLYP/aug-cc-pVDZ//(U)BLYP/cc-pVDZ level of theory. Values in parentheses calculated at the (U)BLYP/aug-ccpVDZ//MCSCF(11,11)/cc-pVDZ level of theory. b At the (U)BLYP/ cc-pVDZ level, the Cs structure for the (zwitterionic) singlet state has one imaginary frequency; a (lower energy) C1 stationary point was used instead.
bond lengths than the (U)BPW91 geometries, although the bond angles obtained using the two methods are about the same. The MCSCF geometries show shorter C-H bond lengths and slightly smaller bond angles about dehydrocarbon atoms than either of the DFT methods, but all other bond angles are about the same as those obtained using either DFT method. The MCSCF geometries also show greater localization of the aromatic π bonds. These geometric differences are similar to those noted17 previously for didehydro(iso)quinolinium cations. The high quality of (U)BPW91/cc-pVDZ geometries, in general, has been noted before5c,d,g,24 and derives in part from canceling errors associated with the approximate functional and the relatively modest basis set size.25 This favorable cancellation of errors makes (U)BPW91/cc-pVDZ a very economical choice for computing aromatic monoradical structures. However, the (U)BLYP/cc-pVDZ geometries are of nearly the same quality,5v,26 and we will focus any discussion of geometrical data primarily on results obtained at the (U)BLYP level because this method has been shown to provide values for EAs that are in good agreement with experimentally determined values (vide infra). Geometries for the (zwitterionic) singlet states of the D(I)Qs were optimized at the RBLYP level only. Interestingly, minimum energy structures for six of the 14 D(I)Q singlet states (i.e., 3-DQ, 5-DQ, 5-DIQ, 6-DIQ, 7-DIQ, and 8-DIQ) were found to be nonplanar. A careful analysis of the geometries of these molecules provides no insight into why some of the singlet states are nonplanar and others are planar. However, an analysis of the partial atomic charges for these molecules suggests that the deviation from planarity permits mixing of the σ and π orbitals and thus allows the contribution of a resonance structure where the dehydrocarbon atom is a carbene, and the charge on nitrogen is annihilated. Even then, it is not clear why this is the case for only 6 of the 14 D(I)Qs, but the difference in energy between the nonplanar and planar structures is quite small (
